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3 | <!-- saved from http://www.win.tue.nl/~aeb/linux/lk/lk-10.html -->
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4 | <meta name="GENERATOR" content="SGML-Tools 1.0.9"><title>The Linux kernel: Processes</title>
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5 | </head>
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6 | <body>
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7 | <hr>
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8 | <h2><a name="s10">10. Processes</a></h2>
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9 |
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10 | <p>Before looking at the Linux implementation, first a general Unix
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11 | description of threads, processes, process groups and sessions.
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12 | </p><p>
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13 | (See also <a href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap11.html">General Terminal Interface</a>)
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14 | </p><p>A session contains a number of process groups, and a process group
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15 | contains a number of processes, and a process contains a number
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16 | of threads.
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17 | </p><p>A session can have a controlling tty.
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18 | At most one process group in a session can be a foreground process group.
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19 | An interrupt character typed on a tty ("Teletype", i.e., terminal)
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20 | causes a signal to be sent to all members of the foreground process group
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21 | in the session (if any) that has that tty as controlling tty.
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22 | </p><p>All these objects have numbers, and we have thread IDs, process IDs,
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23 | process group IDs and session IDs.
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24 | </p><p>
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25 | </p><h2><a name="ss10.1">10.1 Processes</a>
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26 | </h2>
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27 |
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28 | <p>
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29 | </p><h3>Creation</h3>
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30 |
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31 | <p>A new process is traditionally started using the <code>fork()</code>
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32 | system call:
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33 | </p><blockquote>
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34 | <pre>pid_t p;
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35 |
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36 | p = fork();
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37 | if (p == (pid_t) -1)
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38 | /* ERROR */
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39 | else if (p == 0)
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40 | /* CHILD */
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41 | else
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42 | /* PARENT */
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43 | </pre>
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44 | </blockquote>
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45 | <p>This creates a child as a duplicate of its parent.
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46 | Parent and child are identical in almost all respects.
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47 | In the code they are distinguished by the fact that the parent
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48 | learns the process ID of its child, while <code>fork()</code>
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49 | returns 0 in the child. (It can find the process ID of its
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50 | parent using the <code>getppid()</code> system call.)
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51 | </p><p>
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52 | </p><h3>Termination</h3>
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53 |
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54 | <p>Normal termination is when the process does
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55 | </p><blockquote>
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56 | <pre>exit(n);
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57 | </pre>
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58 | </blockquote>
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59 |
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60 | or
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61 | <blockquote>
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62 | <pre>return n;
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63 | </pre>
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64 | </blockquote>
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65 |
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66 | from its <code>main()</code> procedure. It returns the single byte <code>n</code>
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67 | to its parent.
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68 | <p>Abnormal termination is usually caused by a signal.
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69 | </p><p>
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70 | </p><h3>Collecting the exit code. Zombies</h3>
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71 |
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72 | <p>The parent does
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73 | </p><blockquote>
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74 | <pre>pid_t p;
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75 | int status;
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76 |
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77 | p = wait(&status);
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78 | </pre>
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79 | </blockquote>
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80 |
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81 | and collects two bytes:
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82 | <p>
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83 | <figure>
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84 | <eps file="absent">
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85 | <img src="ctty_files/exit_status.png">
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86 | </eps>
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87 | </figure></p><p>A process that has terminated but has not yet been waited for
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88 | is a <i>zombie</i>. It need only store these two bytes:
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89 | exit code and reason for termination.
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90 | </p><p>On the other hand, if the parent dies first, <code>init</code> (process 1)
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91 | inherits the child and becomes its parent.
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92 | </p><p>
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93 | </p><h3>Signals</h3>
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94 |
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95 | <p>
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96 | </p><h3>Stopping</h3>
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97 |
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98 | <p>Some signals cause a process to stop:
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99 | <code>SIGSTOP</code> (stop!),
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100 | <code>SIGTSTP</code> (stop from tty: probably ^Z was typed),
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101 | <code>SIGTTIN</code> (tty input asked by background process),
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102 | <code>SIGTTOU</code> (tty output sent by background process, and this was
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103 | disallowed by <code>stty tostop</code>).
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104 | </p><p>Apart from ^Z there also is ^Y. The former stops the process
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105 | when it is typed, the latter stops it when it is read.
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106 | </p><p>Signals generated by typing the corresponding character on some tty
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107 | are sent to all processes that are in the foreground process group
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108 | of the session that has that tty as controlling tty. (Details below.)
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109 | </p><p>If a process is being traced, every signal will stop it.
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110 | </p><p>
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111 | </p><h3>Continuing</h3>
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112 |
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113 | <p><code>SIGCONT</code>: continue a stopped process.
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114 | </p><p>
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115 | </p><h3>Terminating</h3>
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116 |
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117 | <p><code>SIGKILL</code> (die! now!),
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118 | <code>SIGTERM</code> (please, go away),
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119 | <code>SIGHUP</code> (modem hangup),
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120 | <code>SIGINT</code> (^C),
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121 | <code>SIGQUIT</code> (^\), etc.
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122 | Many signals have as default action to kill the target.
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123 | (Sometimes with an additional core dump, when such is
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124 | allowed by rlimit.)
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125 | The signals <code>SIGCHLD</code> and <code>SIGWINCH</code>
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126 | are ignored by default.
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127 | All except <code>SIGKILL</code> and <code>SIGSTOP</code> can be
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128 | caught or ignored or blocked.
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129 | For details, see <code>signal(7)</code>.
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130 | </p><p>
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131 | </p><h2><a name="ss10.2">10.2 Process groups</a>
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132 | </h2>
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133 |
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134 | <p>Every process is member of a unique <i>process group</i>,
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135 | identified by its <i>process group ID</i>.
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136 | (When the process is created, it becomes a member of the process group
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137 | of its parent.)
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138 | By convention, the process group ID of a process group
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139 | equals the process ID of the first member of the process group,
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140 | called the <i>process group leader</i>.
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141 | A process finds the ID of its process group using the system call
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142 | <code>getpgrp()</code>, or, equivalently, <code>getpgid(0)</code>.
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143 | One finds the process group ID of process <code>p</code> using
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144 | <code>getpgid(p)</code>.
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145 | </p><p>One may use the command <code>ps j</code> to see PPID (parent process ID),
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146 | PID (process ID), PGID (process group ID) and SID (session ID)
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147 | of processes. With a shell that does not know about job control,
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148 | like <code>ash</code>, each of its children will be in the same session
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149 | and have the same process group as the shell. With a shell that knows
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150 | about job control, like <code>bash</code>, the processes of one pipeline, like
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151 | </p><blockquote>
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152 | <pre>% cat paper | ideal | pic | tbl | eqn | ditroff > out
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153 | </pre>
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154 | </blockquote>
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155 |
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156 | form a single process group.
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157 | <p>
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158 | </p><h3>Creation</h3>
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159 |
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160 | <p>A process <code>pid</code> is put into the process group <code>pgid</code> by
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161 | </p><blockquote>
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162 | <pre>setpgid(pid, pgid);
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163 | </pre>
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164 | </blockquote>
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165 |
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166 | If <code>pgid == pid</code> or <code>pgid == 0</code> then this creates
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167 | a new process group with process group leader <code>pid</code>.
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168 | Otherwise, this puts <code>pid</code> into the already existing
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169 | process group <code>pgid</code>.
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170 | A zero <code>pid</code> refers to the current process.
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171 | The call <code>setpgrp()</code> is equivalent to <code>setpgid(0,0)</code>.
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172 | <p>
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173 | </p><h3>Restrictions on setpgid()</h3>
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174 |
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175 | <p>The calling process must be <code>pid</code> itself, or its parent,
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176 | and the parent can only do this before <code>pid</code> has done
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177 | <code>exec()</code>, and only when both belong to the same session.
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178 | It is an error if process <code>pid</code> is a session leader
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179 | (and this call would change its <code>pgid</code>).
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180 | </p><p>
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181 | </p><h3>Typical sequence</h3>
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182 |
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183 | <p>
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184 | </p><blockquote>
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185 | <pre>p = fork();
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186 | if (p == (pid_t) -1) {
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187 | /* ERROR */
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188 | } else if (p == 0) { /* CHILD */
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189 | setpgid(0, pgid);
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190 | ...
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191 | } else { /* PARENT */
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192 | setpgid(p, pgid);
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193 | ...
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194 | }
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195 | </pre>
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196 | </blockquote>
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197 |
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198 | This ensures that regardless of whether parent or child is scheduled
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199 | first, the process group setting is as expected by both.
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200 | <p>
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201 | </p><h3>Signalling and waiting</h3>
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202 |
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203 | <p>One can signal all members of a process group:
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204 | </p><blockquote>
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205 | <pre>killpg(pgrp, sig);
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206 | </pre>
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207 | </blockquote>
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208 | <p>One can wait for children in ones own process group:
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209 | </p><blockquote>
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210 | <pre>waitpid(0, &status, ...);
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211 | </pre>
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212 | </blockquote>
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213 |
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214 | or in a specified process group:
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215 | <blockquote>
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216 | <pre>waitpid(-pgrp, &status, ...);
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217 | </pre>
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218 | </blockquote>
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219 | <p>
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220 | </p><h3>Foreground process group</h3>
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221 |
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222 | <p>Among the process groups in a session at most one can be
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223 | the <i>foreground process group</i> of that session.
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224 | The tty input and tty signals (signals generated by ^C, ^Z, etc.)
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225 | go to processes in this foreground process group.
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226 | </p><p>A process can determine the foreground process group in its session
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227 | using <code>tcgetpgrp(fd)</code>, where <code>fd</code> refers to its
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228 | controlling tty. If there is none, this returns a random value
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229 | larger than 1 that is not a process group ID.
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230 | </p><p>A process can set the foreground process group in its session
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231 | using <code>tcsetpgrp(fd,pgrp)</code>, where <code>fd</code> refers to its
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232 | controlling tty, and <code>pgrp</code> is a process group in
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233 | its session, and this session still is associated to the controlling
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234 | tty of the calling process.
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235 | </p><p>How does one get <code>fd</code>? By definition, <code>/dev/tty</code>
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236 | refers to the controlling tty, entirely independent of redirects
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237 | of standard input and output. (There is also the function
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238 | <code>ctermid()</code> to get the name of the controlling terminal.
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239 | On a POSIX standard system it will return <code>/dev/tty</code>.)
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240 | Opening the name of the
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241 | controlling tty gives a file descriptor <code>fd</code>.
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242 | </p><p>
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243 | </p><h3>Background process groups</h3>
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244 |
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245 | <p>All process groups in a session that are not foreground
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246 | process group are <i>background process groups</i>.
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247 | Since the user at the keyboard is interacting with foreground
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248 | processes, background processes should stay away from it.
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249 | When a background process reads from the terminal it gets
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250 | a SIGTTIN signal. Normally, that will stop it, the job control shell
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251 | notices and tells the user, who can say <code>fg</code> to continue
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252 | this background process as a foreground process, and then this
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253 | process can read from the terminal. But if the background process
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254 | ignores or blocks the SIGTTIN signal, or if its process group
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255 | is orphaned (see below), then the read() returns an EIO error,
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256 | and no signal is sent. (Indeed, the idea is to tell the process
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257 | that reading from the terminal is not allowed right now.
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258 | If it wouldn't see the signal, then it will see the error return.)
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259 | </p><p>When a background process writes to the terminal, it may get
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260 | a SIGTTOU signal. May: namely, when the flag that this must happen
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261 | is set (it is off by default). One can set the flag by
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262 | </p><blockquote>
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263 | <pre>% stty tostop
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264 | </pre>
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265 | </blockquote>
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266 |
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267 | and clear it again by
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268 | <blockquote>
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269 | <pre>% stty -tostop
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270 | </pre>
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271 | </blockquote>
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272 |
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273 | and inspect it by
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274 | <blockquote>
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275 | <pre>% stty -a
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276 | </pre>
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277 | </blockquote>
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278 |
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279 | Again, if TOSTOP is set but the background process ignores or blocks
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280 | the SIGTTOU signal, or if its process group is orphaned (see below),
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281 | then the write() returns an EIO error, and no signal is sent.
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282 | [vda: correction. SUS says that if SIGTTOU is blocked/ignored, write succeeds. ]
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283 | <p>
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284 | </p><h3>Orphaned process groups</h3>
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285 |
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286 | <p>The process group leader is the first member of the process group.
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287 | It may terminate before the others, and then the process group is
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288 | without leader.
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289 | </p><p>A process group is called <i>orphaned</i> when <i>the
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290 | parent of every member is either in the process group
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291 | or outside the session</i>.
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292 | In particular, the process group of the session leader
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293 | is always orphaned.
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294 | </p><p>If termination of a process causes a process group to become
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295 | orphaned, and some member is stopped, then all are sent first SIGHUP
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296 | and then SIGCONT.
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297 | </p><p>The idea is that perhaps the parent of the process group leader
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298 | is a job control shell. (In the same session but a different
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299 | process group.) As long as this parent is alive, it can
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300 | handle the stopping and starting of members in the process group.
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301 | When it dies, there may be nobody to continue stopped processes.
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302 | Therefore, these stopped processes are sent SIGHUP, so that they
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303 | die unless they catch or ignore it, and then SIGCONT to continue them.
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304 | </p><p>Note that the process group of the session leader is already
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305 | orphaned, so no signals are sent when the session leader dies.
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306 | </p><p>Note also that a process group can become orphaned in two ways
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307 | by termination of a process: either it was a parent and not itself
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308 | in the process group, or it was the last element of the process group
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309 | with a parent outside but in the same session.
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310 | Furthermore, that a process group can become orphaned
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311 | other than by termination of a process, namely when some
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312 | member is moved to a different process group.
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313 | </p><p>
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314 | </p><h2><a name="ss10.3">10.3 Sessions</a>
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315 | </h2>
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316 |
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317 | <p>Every process group is in a unique <i>session</i>.
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318 | (When the process is created, it becomes a member of the session
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319 | of its parent.)
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320 | By convention, the session ID of a session
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321 | equals the process ID of the first member of the session,
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322 | called the <i>session leader</i>.
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323 | A process finds the ID of its session using the system call
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324 | <code>getsid()</code>.
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325 | </p><p>Every session may have a <i>controlling tty</i>,
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326 | that then also is called the controlling tty of each of
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327 | its member processes.
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328 | A file descriptor for the controlling tty is obtained by
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329 | opening <code>/dev/tty</code>. (And when that fails, there was no
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330 | controlling tty.) Given a file descriptor for the controlling tty,
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331 | one may obtain the SID using <code>tcgetsid(fd)</code>.
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332 | </p><p>A session is often set up by a login process. The terminal
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333 | on which one is logged in then becomes the controlling tty
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334 | of the session. All processes that are descendants of the
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335 | login process will in general be members of the session.
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336 | </p><p>
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337 | </p><h3>Creation</h3>
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338 |
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339 | <p>A new session is created by
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340 | </p><blockquote>
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341 | <pre>pid = setsid();
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342 | </pre>
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343 | </blockquote>
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344 |
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345 | This is allowed only when the current process is not a process group leader.
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346 | In order to be sure of that we fork first:
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347 | <blockquote>
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348 | <pre>p = fork();
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349 | if (p) exit(0);
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350 | pid = setsid();
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351 | </pre>
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352 | </blockquote>
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353 |
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354 | The result is that the current process (with process ID <code>pid</code>)
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355 | becomes session leader of a new session with session ID <code>pid</code>.
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356 | Moreover, it becomes process group leader of a new process group.
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357 | Both session and process group contain only the single process <code>pid</code>.
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358 | Furthermore, this process has no controlling tty.
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359 | <p>The restriction that the current process must not be a process group leader
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360 | is needed: otherwise its PID serves as PGID of some existing process group
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361 | and cannot be used as the PGID of a new process group.
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362 | </p><p>
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363 | </p><h3>Getting a controlling tty</h3>
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364 |
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365 | <p>How does one get a controlling terminal? Nobody knows,
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366 | this is a great mystery.
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367 | </p><p>The System V approach is that the first tty opened by the process
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368 | becomes its controlling tty.
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369 | </p><p>The BSD approach is that one has to explicitly call
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370 | </p><blockquote>
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371 | <pre>ioctl(fd, TIOCSCTTY, 0/1);
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372 | </pre>
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373 | </blockquote>
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374 |
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375 | to get a controlling tty.
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376 | <p>Linux tries to be compatible with both, as always, and this
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377 | results in a very obscure complex of conditions. Roughly:
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378 | </p><p>The <code>TIOCSCTTY</code> ioctl will give us a controlling tty,
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379 | provided that (i) the current process is a session leader,
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380 | and (ii) it does not yet have a controlling tty, and
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381 | (iii) maybe the tty should not already control some other session;
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382 | if it does it is an error if we aren't root, or we steal the tty
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383 | if we are all-powerful.
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384 | [vda: correction: third parameter controls this: if 1, we steal tty from
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385 | any such session, if 0, we don't steal]
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386 | </p><p>Opening some terminal will give us a controlling tty,
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387 | provided that (i) the current process is a session leader, and
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388 | (ii) it does not yet have a controlling tty, and
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389 | (iii) the tty does not already control some other session, and
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390 | (iv) the open did not have the <code>O_NOCTTY</code> flag, and
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391 | (v) the tty is not the foreground VT, and
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392 | (vi) the tty is not the console, and
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393 | (vii) maybe the tty should not be master or slave pty.
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394 | </p><p>
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395 | </p><h3>Getting rid of a controlling tty</h3>
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396 |
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397 | <p>If a process wants to continue as a daemon, it must detach itself
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398 | from its controlling tty. Above we saw that <code>setsid()</code>
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399 | will remove the controlling tty. Also the ioctl TIOCNOTTY does this.
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400 | Moreover, in order not to get a controlling tty again as soon as it
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401 | opens a tty, the process has to fork once more, to assure that it
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402 | is not a session leader. Typical code fragment:
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403 | </p><p>
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404 | </p><pre> if ((fork()) != 0)
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405 | exit(0);
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406 | setsid();
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407 | if ((fork()) != 0)
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408 | exit(0);
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409 | </pre>
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410 | <p>See also <code>daemon(3)</code>.
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411 | </p><p>
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412 | </p><h3>Disconnect</h3>
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413 |
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414 | <p>If the terminal goes away by modem hangup, and the line was not local,
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415 | then a SIGHUP is sent to the session leader.
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416 | Any further reads from the gone terminal return EOF.
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417 | (Or possibly -1 with <code>errno</code> set to EIO.)
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418 | </p><p>If the terminal is the slave side of a pseudotty, and the master side
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419 | is closed (for the last time), then a SIGHUP is sent to the foreground
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420 | process group of the slave side.
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421 | </p><p>When the session leader dies, a SIGHUP is sent to all processes
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422 | in the foreground process group. Moreover, the terminal stops being
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423 | the controlling terminal of this session (so that it can become
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424 | the controlling terminal of another session).
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425 | </p><p>Thus, if the terminal goes away and the session leader is
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426 | a job control shell, then it can handle things for its descendants,
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427 | e.g. by sending them again a SIGHUP.
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428 | If on the other hand the session leader is an innocent process
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429 | that does not catch SIGHUP, it will die, and all foreground processes
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430 | get a SIGHUP.
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431 | </p><p>
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432 | </p><h2><a name="ss10.4">10.4 Threads</a>
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433 | </h2>
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434 |
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435 | <p>A process can have several threads. New threads (with the same PID
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436 | as the parent thread) are started using the <code>clone</code> system
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437 | call using the <code>CLONE_THREAD</code> flag. Threads are distinguished
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438 | by a <i>thread ID</i> (TID). An ordinary process has a single thread
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439 | with TID equal to PID. The system call <code>gettid()</code> returns the
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440 | TID. The system call <code>tkill()</code> sends a signal to a single thread.
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441 | </p><p>Example: a process with two threads. Both only print PID and TID and exit.
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442 | (Linux 2.4.19 or later.)
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443 | </p><pre>% cat << EOF > gettid-demo.c
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444 | #include <unistd.h>
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445 | #include <sys/types.h>
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446 | #define CLONE_SIGHAND 0x00000800
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447 | #define CLONE_THREAD 0x00010000
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448 | #include <linux/unistd.h>
|
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449 | #include <errno.h>
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450 | _syscall0(pid_t,gettid)
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451 |
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452 | int thread(void *p) {
|
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453 | printf("thread: %d %d\n", gettid(), getpid());
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454 | }
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455 |
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456 | main() {
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457 | unsigned char stack[4096];
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458 | int i;
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459 |
|
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460 | i = clone(thread, stack+2048, CLONE_THREAD | CLONE_SIGHAND, NULL);
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461 | if (i == -1)
|
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462 | perror("clone");
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463 | else
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464 | printf("clone returns %d\n", i);
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465 | printf("parent: %d %d\n", gettid(), getpid());
|
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466 | }
|
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467 | EOF
|
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468 | % cc -o gettid-demo gettid-demo.c
|
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469 | % ./gettid-demo
|
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470 | clone returns 21826
|
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471 | parent: 21825 21825
|
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472 | thread: 21826 21825
|
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473 | %
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474 | </pre>
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475 | <p>
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476 | </p><p>
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477 | </p><hr>
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478 |
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479 | </body></html>
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